1. Field of the Invention
The present invention relates to a battery management system. More particularly, the present invention relates to a state of charge (SOC) compensation method for compensating an SOC of a battery used for a vehicle using electrical energy, and a battery management system using the SOC compensation method.
2. Description of the Related Art
Vehicles using an internal combustion engine of gasoline or heavy oil have caused serious air pollution. To reduce air pollution, various recent efforts for developing electric or hybrid vehicles have been made.
An electric vehicle uses a battery motor operating with electrical energy output by a battery. The electric vehicle mainly uses a battery including a battery pack formed from multiple rechargeable/dischargeable secondary cells. Therefore, it has no emission gases and less noise.
A hybrid vehicle commonly refers to a gasoline-electric hybrid vehicle that uses gasoline to power an internal-combustion engine and an electric battery to power an electric motor. Recently, hybrid vehicles using both an internal-combustion engine and fuel cells and hybrid vehicles using both a battery and fuel cells have been developed. The fuel cells directly obtain electrical energy by generating a chemical reaction while hydrogen and oxygen are continuously provided.
A vehicle using electrical energy charges a battery with residual power from operating an electric generator when power output from the engine is greater than the power needed for driving the vehicle. In turn, when output from the engine is low, the vehicle operates a motor by using battery power to output power. During this time, the battery is discharged.
Since battery performance directly affects a vehicle using electrical energy, good performance is required from each battery cell. Also, it is required to provide a battery management system for measuring voltage and current of the overall battery to efficiently manage charging/discharging operations of each battery cell.
Accordingly, a conventional battery management system (BMS) detects voltage, current, and temperature of a rechargeable battery to estimate the SOC of the battery, and controls the SOC to optimize fuel consumption efficiency.
To perform a stable power assist operation for driving a motor used in a vehicle during acceleration of the vehicle and a stable energy recovery operation (regenerative braking) during deceleration of the vehicle, the BMS controls the SOC level of the battery. The SOC level is controlled to prevent overcharging the battery when the SOC level approaches 70%, and to prevent over-discharge of the battery when the SOC level approaches 50%. That is, the BMS controls the SOC level to be within a range between 50% and 70%.
To control the SOC level, it is required to have a substantially accurate estimate of the SOC level of the charged and discharged battery.
Conventional methods for estimating the SOC level are classified into an SOC estimation method using an integrated current and an SOC estimation method using an open circuit voltage (OCV). In the SOC estimation method using the integrated current, a charge and discharge current value is measured, a value obtained by multiplying the measured current value by a charging efficiency is integrated over a predetermined duration of time to calculate an integrated charging capacity of the battery, and the SOC level is estimated based on the integrated charging capacity. The charge and discharge current is the current generated by the battery during the time the battery is being charged or during the time it is being discharged.
In the SOC estimation method using the OCV, a pair of data including the charge and discharge current and a rechargeable battery terminal voltage corresponding to the charge and discharge current are measured and stored. A line is fitted to the pairs of data that include the voltage V versus the current I using a least square method of curve fitting. The voltage value V0 corresponding to a current value 0, in other words the V intercept of the V-I line, is calculated as an OCV corresponding to the current 0. Finally, the SOC level is estimated based on the OCV of the battery.
However, in the SOC estimation method using the integrated current method, since the charging efficiency required for integrating the current value depends on the SOC level, current value, and temperature, it is difficult to detect the charging efficiency adapted to various conditions. It is also difficult to calculate a self-discharge state when the battery is not being used.
Further, with passing time, a difference between true and estimated values of the SOC is increased in the SOC estimation method using the integrated current, and the SOC level may not be accurately estimated.